The Medium Access Control Sublayer. Chapter 4. The Channel Allocation Problem. Static Channel Allocation in LANs and MANs Dynamic Channel Allocation in LANs and MANs. Dynamic Channel Allocation in LANs and MANs. Station Model. Single Channel Assumption. Collision Assumption.
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Pure ALOHA In pure ALOHA, frames are transmitted at completely arbitrary times. CS522 Tanenbaum
Pure ALOHA (2) Vulnerable period for the shaded frame. CS522 Tanenbaum
Pure ALOHA (3) A station transmits a frame whenever it has data to send. When the frame collides with other frame(s), retransmit it by waiting a random amount of time. What is the throughput (or efficiency) of an ALOHA channel? Let N (no. of stations using an ALOHA channel) = ∞ , assume infinite population, new arrival is a Poisson process, λn = mean new frame arrival rate (frame/sec), µ = channel capacity (frame/sec), T=1/ µ=frame time (sec/frame) S = λn/µ= λn (frame/sec). 1/µ (sec/frame) = mean new frames per frame time, S is traffic density or channel utilization, λr = mean retransmitted frame rate. The combined traffic of new and old (retransmission) traffic is still a Poisson process with G= mean departure frames per frame time=(λn+λr)/µ=λT. G is actual traffic density. G can be controlled by adjusting retransmission time. Can S be controlled? What is the system throughput? S or G? Analysis: • If S>1, almost every frame will collide. • For reasonable throughput, 0<S<1. G > S. • At low load, S ≈ 0 → G ≈ S. • At high load, G>S. CS522 Tanenbaum
Pure ALOHA (4) • What is the relationship between G and S? S = GP0P0 = the probability that a frame does not suffer a collision. • How to find P0?Note that the combined frame traffic is a Poisson process, therefore,Pr[k] = the probability that k frames are generated during a frame time T = (λ=λn+λr, T=1/µ → λT=G substitute λT with G ) = • But observe Figure 4-2, the vulnerable period of a frame is 2 frame time. • Pr’[k] = (T’=2T, λT’=2G) • P0 = Pr’[0] = . • Observation:S depends on incoming traffic.G depends on S. For each S, there are two G values in Figure 4-3. CS522 Tanenbaum
What is the max. throughput of Pure ALOHA? How to control G to get maximum S? Let , Smax = The best we can hope for the channel utilization is 18.4%. Is 18.4% channel utilization for the ALOHA system too low? For a terminal user sends 60 character/msg every 2 min., input rate is 0.5 char/sec = 5 bits/sec (assume 10 bit async. transmission) For 4800 bps channel and 10% utilization, it can support 96 interactive users. For bursty, interactive traffic, pure ALOHA is sufficient and simple. CS522 Tanenbaum
Slotted ALOHA Transmission time is divided into slots. Stations with data to transmit will wait until the starting time of a slot. Time of vulnerable period is reduced to 1 frame (slot) time. (Require synchronization devices, e.g., a central station broadcasts the clock signal to all stations for synchronizing the slots.) Pr[k] = P0 = Pr[0] = e-G S=GP0=Ge–G. Let , Smax = CS522 Tanenbaum
Carrier Sense MA protocols Protocols in which stations listen for a carrier (i.e. a transmission medium) and act accordingly, e.g. MA protocols used by LANs. 1-Persistent CSMA When a station has data to send, it listens to the channel. If the channel is busy, it waits until channel idle. When the channel is idle, it transmits a frame (with probability = 1). Propagation delay effect: At t0, station 1 detects idle and sends a frame. At t0+τ−ε, station 2 detects idle and sends a frame.2 At t0+τ, station 2 detects collision. CS522 Tanenbaum
Carrier Sense MA Protocols(2) At t0+τ−ε+τ, station 1 detects collision. In worst case, only about 2τ time later can station 1 detect the collision. The longer the cable, the longer stations have to wait to be sure that there is no collision. Non-persistent CSMA • If the channel is busy, it waits for random period then sense the channel again. • Better channel utilization but longer delays than 1-persistent CSMA. p-persistent CSMA(applies to slotted channel) • If the channel is idle, it transmits with prob = p (with 1-p, it defers until next slot). • If the next slot is idle again, do the same thing. CS522 Tanenbaum
Persistent and Nonpersistent CSMA Comparison of the channel utilization versus load for various random access protocols. Smaller p has better throughput but at what cost? CS522 Tanenbaum
CSMA with Collision Detection CSMA/CD can be in one of three states: contention, transmission, or idle. CS522 Tanenbaum
Broadband Wireless • Comparison of 802.11 and 802.16 • The 802.16 Protocol Stack • The 802.16 Physical Layer • The 802.16 MAC Sublayer Protocol • The 802.16 Frame Structure CS522 Tanenbaum
The 802.16 MAC Sublayer Protocol Service Classes • Constant bit rate service • Real-time variable bit rate service • Non-real-time variable bit rate service • Best efforts service CS522 Tanenbaum
